Interpret flags and operands for completion

internal/commandmeta/flags.go

@@ -0,0 +1,67 @@
+package commandmeta
+
+import "strings"
+
+// FlagTokenKind identifies the portion of the shared option grammar a token uses.
+type FlagTokenKind uint8
+
+const (
+	OperandToken FlagTokenKind = iota
+	TerminatorToken
+	LongFlagToken
+	ShortFlagToken
+)
+
+// ClassifyFlagToken distinguishes option tokens without consuming their values.
+func ClassifyFlagToken(token string) FlagTokenKind {
+	if token == "--" {
+		return TerminatorToken
+	}
+	if strings.HasPrefix(token, "--") && len(token) > 2 {
+		return LongFlagToken
+	}
+	if len(token) > 1 && token[0] == '-' {
+		return ShortFlagToken
+	}
+	return OperandToken
+}
+
+// SplitLongFlag separates the entered spelling from an optional attached value.
+func SplitLongFlag(token string) (spelling, value string, attached bool) {
+	return strings.Cut(token, "=")
+}
+
+// FlagLookup maps canonical and applicable alias spellings to neutral flags.
+type FlagLookup struct {
+	canonical map[string]Flag
+	aliases   map[string]Flag
+}
+
+// NewFlagLookup includes the built-in help flag alongside the supplied flags.
+func NewFlagLookup(flags []Flag, aliases []FlagAlias) FlagLookup {
+	canonical := make(map[string]Flag, len(flags)*2+2)
+	help := HelpFlag()
+	canonical["--help"], canonical["-h"] = help, help
+	for _, flag := range flags {
+		canonical["--"+flag.Name] = flag
+		if flag.Short != "" {
+			canonical["-"+flag.Short] = flag
+		}
+	}
+	lookup := FlagLookup{canonical: canonical, aliases: make(map[string]Flag, len(aliases))}
+	for _, alias := range aliases {
+		if target, ok := canonical[alias.Target]; ok {
+			lookup.aliases[alias.Source] = target
+		}
+	}
+	return lookup
+}
+
+// Find returns the canonical flag for an entered spelling.
+func (lookup FlagLookup) Find(spelling string) (Flag, bool) {
+	flag, ok := lookup.canonical[spelling]
+	if !ok {
+		flag, ok = lookup.aliases[spelling]
+	}
+	return flag, ok
+}